mfd: Add module build support for RZ/G2L MTU3a

Modified Kconfig to enable module build support for RZ/G2L MTU3a driver.
While at it, added module.h header file.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230711092841.119161-1-biju.das.jz@bp.renesas.com


Signed-off-by: Lee Jones <lee@kernel.org>